Abstract

Zinc production has been known since 200 BC. Fluidised bed roasting is the first process stage of the electrolytical zinc production process, was developed in the 1940s.

The raw material for zinc is usually sphalerite concentrate. This sulphide concentrate is oxidised in a roaster. Oxidation reactions produce energy, which is removed as steam, and sulphur dioxide, which is used as the raw material of sulphuric acid.

During recent decades sphalerite concentrates have contained more and more impurities and at the same time they have become more fine-grained. Impurities cause problems during fluidised bed roasting. As a consequence, production capacity decreases, there are breaks in production. Starting up and shutting down a process during production breaks cause the environmental emissions. In order to be able optimise production, the oxidation mechanisms of impure sphalerite and methods for controlling them have to be known.

The hypothesis of this work was as follows: In addition to temperature, the impurity content and particle size of the feed and oxygen coefficient also have an effect on the stability of fluidised bed roasting. Diverse concentrates require different oxygen coefficients and temperatures. The basic target of this work was to develop a method to help find the required conditions and to control them in industrial roasters. This study was restricted to considering the effects of the iron, copper and lead contents in sphalerite concentrate.

A review was made of earlier roasting studies and experiences. This study also evaluated the thermodynamic background of roasting. The oxidation mechanisms were also studied in the laboratory using a fluidised bed roaster and horizontal tube furnace. The results were validated in an industrial roaster.

On the grounds of these studies the different sphalerite concentrates really do require diverse roasting temperatures and oxygen coefficients. Foremore, the same kinds of concentrates require a different roasting temperature and oxygen coefficient, if their particle size distributions are different. Controlling the concentrate feed particle size may help to control the stability of the roasting bed and the temperature of the upper part of the furnace.

The impurities increase the forming of direct bond sintering and thus the forming of sulphide liquid phases. Oxides and sulphates may also form liquid phases. These kinds of liquid phases cause problems in the fluidised bed. Continuous control of the oxygen coefficient and bed temperature and the use of a unique oxygen coefficient and temperature range for every concentrate mixture would make it possible to minimise problems in the furnace.

Laboratory and industrial scale tests have verified the variables and methods for controlling conditions in the roaster bed.